Electrically insulating members, especially electrically insulating enclosures, used in power cable connections such as intermediate connections between power cables and terminations between power cable terminals and terminal fittings are often made of elastic rubbers having good electrical insulation such as silicone rubbers and EP rubbers.
Three different methods are generally used for the connection of power cables. A first method uses an insulating enclosure which is slidably moved over the sheath of a power cable to a connection between power cables to form a joint structure. A second method uses a once rolled-up enclosure which is unwrapped over a connection to form a joint structure. In a third method, a core dilator is previously inserted inside an insulating enclosure to keep the enclosure in the dilated state, the core dilator with the dilated enclosure fitted thereon is moved to a connection between power cables, and the core dilator is then drawn out whereupon the enclosure shrinks and fits in place to form a joint structure. The shrinkage of the enclosure occurs at room temperature. Of these methods, the third method or enclosure shrinkage fit method is preferred for operation efficiency.
In particular, the rubber material of which the enclosures of room temperature shrinkage type are made must be readily dilatable, be minimized in tensile permanent set which is a measure of restoration after removal of the core dilator, and have high tensile strength, high tear strength and excellent tracking resistance which is a measure of electrical insulation. The required physical properties of rubber include, for instance, a tensile strength of at least 8 MPa, a tear strength of at least 25 N/mm, an elongation at break of at least 600%, a tensile permanent set (JIS K6249) of up to 20 under conditions: 180° C., 22 hours and 100% elongation, which are regarded as the target values.
When a comparison is made between silicone rubber and ethylene-propylene (EP) rubber, the silicone rubber has a better restoring ability on account of a lower tensile permanent set so that after removal of the core dilator, an enclosure thereof establishes a better seal and closer contact to the cable or the like. Another advantage is that the silicone rubber can be formulated into a higher elongation material which is easy to dilate.
The silicone rubber, however, suffers from a problem that on outdoor use, the silicone rubber is susceptible to tracking by leakage current and is degraded thereby. To improve tracking resistance to form satisfactory electrically insulating silicone rubber, the silicone rubber must be heavily loaded with aluminum hydroxide, which undesirably degrades the mechanical strength of rubber, below the target rubber physical properties described above.
JP-A 61-228062 describes a flame retardant silicone rubber composition comprising an organosilicon compound having a nitrogenous organic group and an unsaturated group and a platinum compound. This composition has improved flame retardance, but lacks some properties like mechanical strength and is thus unsuitable for power cable connection. JP-A 9-284977 describes an enclosure for power cable connection made of a silicone rubber composition comprising aluminum hydroxide, benzotriazole and a platinum compound. Because of essential inclusion of aluminum hydroxide, this composition has poor mechanical strength.